This invention relates to olefin upgrading by fluidized bed catalysis.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by medium pore ZSM-5 type shape selective zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks, that contain lower olefins, especially C.sub.2 -C.sub.4 alkenes. Conversions of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the ZSM-5 type zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixtures with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Conversion of lower olefins, especially propene (propylene) and butenes, over HZSM-5 is effective at moderately elevated temperatures and pressures. The conversion products are sought as liquid fuels, especially the C.sub.5 + aliphatic and aromatic hydrocarbons. Product distribution for liquid hydrocarbons can be varied by controlling process conditions, such as temperature, pressure and space velocity. Gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperature--e.g., from about 250.degree. C. to 700.degree. C. (preferably 300.degree. C. to 500.degree. C.) and moderate pressure from ambient to about 5500 kPa (preferably about 250 to 2900 kPa). Olefinic gasoline can be produced in good yield and may be recovered as a product or fed to a low severity, high pressure reactor system for further conversion to heavier distillate-range products.
Distillate mode operation can be employed to maximize production of C.sub.10 + aliphatics by reacting the lower and intermediate olefins at high pressure and moderate temperature. Operating details for typical "MOGD" oligomerization units are disclosed in U.S. Pat. Nos. 4,456,779; 4,497,968 (Owen et al) and 4,433,185 (Tabak), incorporated herein by reference. At moderate temperature and relatively high pressure, the conversion conditions favor distillate-range product having a boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however, the low severity distillate mode conditions do not convert a major fraction of ethene (ethylene). While propene, butene-1, and others may be converted to the extent of 70% to 99% in the lower severity moderate temperature distillate mode, only about 10% to 30% of the ethene component will be converted using HZSM-5 or similar acid zeolites. Many feedstocks of commercial interest, such as FCC offgas, dehydrogenation products, ethane cracking by-products, etc., contain both ethene and hydrogen along with H.sub.2 S and light aliphatics. Ethene can also be converted at moderate temperature with a bifunctional nickel catalyst. In conventional processes of upgrading FCC light gas containing amounts of ethene, propene and butenes; the entire FCC fuel gas and C.sub.3 -C.sub.4 product is added directly to a reaction system. Such processes require gas plant fractionation to recover propane and butanes from the reaction system effluent. In a typical operation, about 20% to 70% of the C.sub.3 -C.sub.4 alkanes in the effluent are not easily recoverable. Such a disadvantage is serious because an objective in employing catalytic fuel gas upgrading units is to reduce FCC fuel gas make. It has been found that ethene-containing light gas can be upgraded to liquid hydrocarbons rich in isobutane and gasoline or BTX by catalytic conversion in a turbulent fluidized bed of solid acid zeolite catalyst under high severity reaction conditions in a single pass or with recycle of gas product. This technique is particularly useful for upgrading FCC light gas, which usually contains significant amounts of ethene, propene, paraffins and hydrogen produced in cracking heavy petroleum oils and the like. By upgrading the by-product light gas, gasoline yield of FCC units can be significantly increased. Accordingly, it is a primary object of the present invention to provide a novel technique for upgrading ethene-containing light gas. U.S. Pat. No. 4,090,949 (Owen et al) discloses dual riser reactors operating in parallel and containing zeolite catalyst particles obtained from a common regeneration zone. A gas oil feed material is contacted with catalyst in a first riser reactor and a C.sub.2 -C.sub.5 olefinic stream is contacted with catalyst in a second riser reactor. Hydrogen contributors are injected into both riser reactors at multiple positions along each riser.
U.S. Pat. No. 4,456,779 (Owen et al) discloses a process for upgrading C.sub.2 -C.sub.5 olefinic feedstocks in a reaction zone containing zeolite ZSM-5 catalyst particles. The feedstock is mixed initially with a recycled C.sub.2 -C.sub.4 paraffinic material obtained from a debutanizer.
U.S. Pat. No. 4,487,985 (Tabak) discloses a reactor sequencing technique useful for multi-stage hydrocarbon conversion systems. Fixed bed catalytic reactors are employed. Catalyst partially inactivated in a primary stage is employed in a secondary stage to effect hydrocarbon conversion at a higher temperature.
U.S. Pat. No. 4,497,968 (Wright et al) discloses a multistage process for converting lower olefins to gasoline boiling range hydrocarbons. An olefinic feedstock is prefractionated to obtain an ethene containing stream and a stream comprising C.sub.3 + olefins such as propene, butene and the like. The ethene-containing stream is added to a high severity reaction system and the C.sub.3 + olefinic stream is added to a distillate mode reaction system. In a preferred embodiment, the olefinic feedstock is obtained from an oxygenates conversion reaction system such as the methanol to-olefins (MTO) process. The distillate mode reaction system preferably comprises a series of fixed bed reactors.
U.S. Pat. No. 4,831,203 (Owen et al) discloses a fluidized bed process for upgrading ethene-containing hydrocarbons to gasoline products. All of the references listed above are incorporated herein by reference.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as catalyst having the structure of ZSM-5, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor distillate range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.6 alkenes may be converted selectively; however, the distillate mode conditions do not convert a major fraction of ethylene.
In the gasoline mode, ethylene and the other lower olefins are catalytically oligomerized at higher temperature and moderate pressure. However, coking of the catalyst is accelerated by the higher temperature. Under these conditions ethylene conversion rate is greatly increased and lower olefin conversion is nearly complete to produce an olefinic gasoline comprising pentane, pentene and C.sub.6 + hydrocarbons in good yield. To avoid excessive temperatures in the exothermic reactors, the lower olefinic feed may be diluted. In the distillate mode operation, olefinic gasoline may be recycled and further oligomerized, as disclosed in U.S. Pat. No. 4,211,640 (Garwood and Lee). In either mode, the diluent may contain light hydrocarbons, such as C.sub.3 -C.sub.4 alkanes, present in the feedstock and/or recycled from the debutanized product. In U.S. Pat. No. 4,433,185 (Tabak), incorporated herein by reference, a two stage catalytic process is disclosed for converting lower olefins at elevated temperature and pressure, with unconverted reactant, mainly ethylene, from a first stage being completely converted at higher temperature in a second stage. Although, the same type catalyst (H-ZSM-5) is employed in each stage, significant differences in operating temperatures and catalyst use contribute to different rates of inactivation, largely due to coking.
The present invention takes advantage of the accelerated aging rate for hydrocarbon conversion catalyst operating under process conditions which produce coke deposits. Increased coking decreases conversion at a given temperature, and it is conventional practice to increase process temperature to maintain the desired level of conversion. In the olefins oligomerization process contemplated in the preferred embodiment herein, partially deactivated coked catalyst from a high severity reaction zone is transferred to a low severity reaction zone operating in parallel. The partially deactivated catalyst retains enough acid activity to convert more reactive olefins under low severity conditions. In a preferred embodiment, highly active ZSM-5 type catalyst is used in the high severity reaction zone for conversion of ethylene (C.sub.2 H.sub.4 ethene). When the ZSM-5 type catalyst is deactivated to a point below which efficient conversion of ethene can be achieved, the catalyst or a portion thereof is removed to the low severity reaction zone for contact with more reactive olefins such as propene, butene and the like. Under low severity conditions C.sub.3 + olefinic reactants are efficiently converted in major amount to gasoline product.